Describe the management of obstetric anal sphincter injuries (OASIS) including risk factors, mechanism, prevention, diagnosis, emergency management and definitive management

Clinical overview

Obstetric anal sphincter injuries (OASIS) — third- and fourth-degree perineal tears — are among the most consequential of all delivery complications, because the injury that is missed in the delivery room becomes the woman who returns months later with faecal urgency, flatal incontinence, perineal pain or a rectovaginal fistula. The sphincter complex governs continence; a tear that is not recognised and repaired correctly the first time is far harder to salvage at secondary surgery. For the FCOG(SA) registrar this objective is therefore a discipline of attention: every vaginal birth ends with a deliberate, systematic perineal and rectal examination, because the single greatest determinant of a good outcome is whether the injury is detected at all.

In South Africa, where many vaginal births occur at district hospitals and Midwife Obstetric Units, the registrar carries a dual responsibility — to repair correctly when the woman is in front of you, and to build the local culture (training, protocols, audit) that ensures the midwife conducting an unsupervised delivery recognises and refers an OASIS rather than suturing it as a second-degree tear. OASIS sits alongside Perineal protection and Instrumental delivery as a triad of intrapartum technique that the exam expects you to integrate. The condition is not, in the immediate sense, life-threatening, but it is a "must-not-miss" and a major cause of long-term maternal morbidity and litigation.

Core knowledge

Anatomy of the sphincter complex

Continence depends on two muscular rings around the anal canal. The external anal sphincter (EAS) is striated, voluntary, tonically active and supplied by the pudendal nerve; it provides the squeeze pressure that defers defaecation. The internal anal sphincter (IAS) is smooth muscle, involuntary, a downward continuation of the rectal circular muscle, and provides the majority of resting anal tone. The IAS is the structure most relevant to passive leakage and flatal incontinence, and it is the layer most often overlooked at primary repair. Relate this to Genital anatomy when revising the pelvic floor.

Classification (Sultan / RCOG, adopted by NICE and SA practice)

The internationally accepted classification grades by depth into the sphincter complex:

A "buttonhole" tear of the rectal mucosa with an intact sphincter is a separate entity that must be recognised and repaired in its own right, as it predisposes to fistula. Grading every OASIS precisely matters because it drives the repair technique and the counselling about future delivery.

Tap to enlarge

Figure J10.1 — OASIS grading map showing Sultan/RCOG depth-based classification from first-degree tears through 3a/3b/3c and fourth-degree injury, with IAS, EAS, anal mucosa and buttonhole pitfalls highlighted.

Mechanism of injury

The sphincter is torn when the descending presenting part overstretches and lacerates the posterior perineum as the head (or, in breech, the after-coming head/shoulders) distends the introitus. The forces are greatest at crowning, when the perineum is maximally stretched over the largest diameter. Anything that increases the rate or magnitude of that stretch, or that reduces perineal compliance, increases tear risk: a precipitate uncontrolled delivery, a large fetal head, occipito-posterior position presenting a larger diameter, instrumental traction (particularly forceps), and shoulder dystocia. A midline episiotomy propagates directly toward the anus and is strongly associated with sphincter extension — which is precisely why South African and UK practice favours the mediolateral cut, angled away from the midline (target ~60° from the midline at the point of incision, when the perineum is distended). This mechanistic understanding connects directly to the prevention bundle below and to Perineal protection.